I've been guilty of increasing only the Speed and Stamina of my Chocobo. After all they were the two most important stats right? Well wrong. It might be obvious to some people but I didn't know this at all, obliviously pumping Speed/Stamina and wondering why I wasn't doing remarkably better.
That was until I decided to raise my lagging Endurance. Only then did my overall speed and performance improve in races. I realized too late (P9 now) that I've been messing up my Chocobo all along. Here's what Endurance does:
Assuming you have 200 points to distribute between Speed and Endurance, this is what happens.
If you had increased them both equally, you would squeeze the most amount of speed with the stamina that you have.
If you neglect your Endurance (like I did), you're actually limiting yourself. You'll find that you'll hit lathered state without going any faster.
This is useful for people leveling up their birds. At least you know what stats to put into now!